Modern ducted fan gas turbine engines are provided with a front fan which provides both propulsive thrust and a supply of air for the gas generator core of the engine. Typically such fans comprise a hub having a plurality of generally axially extending grooves in its periphery which receive the roots of the fan blades. The grooves and roots are usually of corresponding generally dovetail cross-section shape so as to ensure radial retention of the fan blades.
It is necessary for assembly and maintenance purposes that each of the fan blades should be easily removable from its respective groove in the hub. One way of achieving this is to provide fixed stops at the rearward ends of the hub grooves which the fan blade roots are slid up to. A retention ring is then bolted on to the front of the hub to ensure that forward motion of the roots in their grooves is prevented. While this method of retaining blades is effective for small to medium size engines, it can be less suitable for large engines because of the weight problem associated with a retention ring which is sufficiently robust to ensure effective blade root retention.
An alternative way of retaining fan blades in their slots is described in GB1523422. In that specification, there is described a fan blade assembly in which the fan blades are axially retained by means of a U-shaped bar. The bar locates in appropriate aligned slots in the blade root and hub to provide axial retention. The blade roots and part of the hub rim are partially extended in an upstream direction so as to accommodate the U-shaped bars. A lip provided on a fairing attached to the front face of the hub cooperates with a ring to maintain the U-shaped bars in position.
While such a method of fan blade retention is effective, the extension of the hub rim and blade roots in an upstream direction does give rise to undesirable weight penalties. Moreover it can be difficult to remove a single fan blade for maintenance purposes without disturbing the remaining fan blades. Additionally there can be difficulty in manipulating the U-shaped bars during the installation and removal of the fan blades.
In our co-pending GB patent application no 9223593.6 there is described an improved fan blade retention system. Each fan blade has radial slots in its root portion which are aligned with corresponding slots in the hub groove which receives the fan blade root portion. The aligned slots accommodate a U-shaped key which prevents relative axial movement between each fan blade and the hub.
While this fan blade retention system is highly effective under normal operating conditions, it can sometimes give rise to hub damage in the event of the fan blade being impacted by a foreign object, such as a bird. If a large foreign object impacts a fan blade, the fan blade root portion may move relative to the groove which receives it. This can place shear loads upon the U-shaped key which in turn can cause damage to the fan blade hub. Such fan blade hub damage is unacceptable in view of the stress and fatigue limitations which it places on the disc.